Ellagic acid as stabilizer in the thermo-oxidative degradation of thermoplastic polyurethane

Abstract

Ellagic acid (EA) is a plant polyphenol with strong antioxidant properties. The stabilizing effect of EA on the thermo-oxidative degradation of the commercial thermoplastic poly(ester-ether urethane) (TPU) was studied for the first time and compared with effect of kraft lignin (KL), butylated hydroxytoluene (BHT) and pentaerythritol tetrakis (Irganox 1010). TPUs doped with antioxidants (ca. 20,000 ppm), were analysed for their oxidation induction time (OIT) at 200 °C and the oxidation onset temperature (OOT). The extent of TPU degradation with and without addition of antioxidants was studied by head space-gas chromatography-mass spectrometry (HS-GC-MS) at 200 °C under aerobic atmosphere. The highest stabilizing effect of EA was attributed primarily to its stronger antioxidant capacity when compared to KL and synthetic polyphenolic antioxidants (BHT and Irganox), which was corroborated by antiradical power studies with 2,2-diphenyl-1-picrylhydrazyl (DPPH). The analysis of the volatile products released under thermo-oxidative degradation showed the suppressive effect of EA on the thermally induced depolymerization of TPU occurring via homolysis. Furthermore, it has also been proposed that EA affects TPU degradation with the cleavage of urethane bonds in carbamates due to eventual coordination with the intermediate cyclic intramolecular transition complex involved in thermally induced depolymerization. Further studies in this area can prompt the performance improvements of TPU products, allowing them to be used in a wider range of durable goods applications.